The invention relates to a fuel metering system having apparatus for regulating the composition of the fuel-air mixture reaching the combustion chambers of an internal combustion engine by means of a sensor disposed in the flow of exhaust gas of the engine.
U.S. Pat. No. 4,073,269, issued Feb. 14, 1978 to Herth et al, describes a fuel metering system for an internal combustion engine having this type of apparatus for regulating the air/fuel ratio of the combustible mixture. The apparatus includes a threshold switch which is connected to receive the output signal of the sensor. The threshold switch output is connected with the input of an integrator which integrates in alternating directions, depending upon the output signal of the threshold switch. The apparatus also includes a delay device, which is connected with the threshold switch output to respond to a changing threshold switch output signal, for delaying the switchover of the integrator from one integrating direction to the other integrating direction for a predetermined, adjustable delay period. These delay periods are provided for this switchover so that a certain shift in the mixture toward "rich" is attained at the optimal operational point of a catalytic converter disposed in the flow of exhaust gas downstream of the sensor.
However, in the case of relatively high-frequency exhaust gas sensor switching cycles, a repeated "setting of the delay period" causes an undesired and uncontrolled shift toward a rich mixture. This, in turn, substantially worsens the exhaust emissions. The higher-frequency switching procedures are caused by individual rich or lean cylinders, and this nonuniformity in the mixture composition can be caused both by the fuel metering and by a pulsation of the aspirated air quantity. A suitable selection of the installation point of the sensor can substantially prevent this effect because of an improved homogenization of the exhaust gas. However, this cannot be realized in all cases because of other peripheral conditions determining the installation of the sensor such as temperature, space available for installation, and sensor response time.